This invention relates to magnetic resonance apparatus.
Such apparatus can be used for non-invasive internal examinations of patients to produce, for example, cross-sectional internal structural images, and blood flow and spectroscopy data.
In use of such apparatus the part of the patient to be examined is placed in a region of strong static magnetic field to define an equilibrium axis of magnetic alignment in the examination region. A radio frequency (RF) magnetic field is then applied temporarily to the examination region, in a direction orthogonal to the static magnetic field direction, to excite magnetic resonance in material, typically hydrogen protons, in the examination region. The resulting RF signals are detected and analyzed. During this sequence of operations one or more gradients are normally imposed on the static magnetic field in the examination region, e.g. to encode spatially the detected RF signals, or for other purposes such as flow encoding.
The various fields applied in use of a magnetic resonance apparatus give rise to appreciable noise which can be disturbing to a patient, especially when the patient is very ill and/or the patient's head is being examined. To overcome this difficulty it has been proposed to use an active noise reduction arrangement. Known active noise reduction arrangements giving the required level of performance comprise a headset incorporating two enclosures which fit respectively around the two ears of the patient. Each enclosure houses a microphone which senses noise in the enclosure. The microphone signal is processed remote from the headset and a resulting signal applied to a loudspeaker in the enclosure to produce sound waves of a phase, amplitude and frequency such as to tend to reduce the noise heard by the patient to a minimum.
One problem which arises with such an arrangement is that the headset interferes with the static and RF magnetic fields required in the examination region, more particularly the required uniformity of such fields.